T cell responses are highly regulated to assure appropriate and effective responses to infectious and inflammatory stimuli. A key component of this is the B7:CD28 receptor family, consisting of activating (CD28 and ICOS) and inhibitory (CTLA-4, PD-1 and BTLA) members and their ligands (B7 proteins). These receptors play a critical role in regulating lung inflammation by determining whether an immune response is initiated, and once initiated, its resolution. The goal of the studies proposed in this application is to define novel mechanisms by which the B7:CD28 family regulates the initiation and resolution of lung inflammation, and determine how this can be manipulated to therapeutic advantage. To study the molecular mechanisms controlling the initiation of inflammation, we developed gene targeted CD28-mutant knockin mice. Using these we established which signaling motifs mediate specific in vitro and in vivo CD28-dependent functions. In contrast to the prevailing dogma that PI3 kinase signaling initiated by the proximal tyrosine (Y170) motif is the critical pathway activated by CD28, we demonstrated that it is in fact dispensable for CD28 function. However, we showed that the distal proline (PYAP) motif initiated a non- redundant signaling pathway required for normal CD28-dependent responses. Despite mutation of this motif, some function remains the biochemical basis for which is unknown. Therefore, we hypothesize that novel pathways activated independent of the distal proline motif regulate CD28-dependent T cell proliferation and cytokine secretion. We further found that signaling initiated by the distal proline motif is essential for T cell: B cell collaboration and T-dependent antibody production. The signaling pathways that mediate these responses will be determined in specific aim 1 of this proposal. Ongoing inflammation can be inhibited by therapeutic manipulation of the B7:CD28 pathway using the recently approved drug CTLA4Ig, (abatacept, OrenciaTM). This drug binds to B7-1 (CD80) and B7-2 (CD86), and has been presumed to function by preventing CD28 engagement on the T cell, thereby preventing a CD28-mediated signal to the T cell. While this mechanism is true for initial T cell activation, we have established that in fact CTLA4Ig inhibits effector responses by a novel CD28-independent, inducible nitric oxide synthase (NOS2)-dependent mechanism. We further show that IFN3/STAT1 signaling is central in this response. We hypothesize that B7 engagement by CTLA4Ig induces INF3 dependent activation of macrophages or myeloid derived suppressor cells (MDSCs), and that these inhibit allergic lung inflammation by a NOS2-dependent mechanism. In this application, we propose cellular, molecular and in vivo studies to determine the mechanism of the novel mode of action for CTLA4Ig., which may have broad applicability to therapeutic manipulation of inflammatory lung disease PUBLIC HEALTH RELEVANCE: This proposal is to study the control of lung inflammation by group of receptors called the B7:CD28 family. These receptors control how the immune system fights infection and are also important in diseases such as asthma. By understanding how these receptors work, we can design new therapies that may help patients with a number of lung diseases. )